We have 20 thru hulls, with seacocks, that I can count off the top of my head.

Nevertheless, let's sound off. Do you close your thru hulls when you leave your boat?

Oh. My. God.

My answer:
The engine seacock is shut whenever I leave the boat. I leave the head's seawater intake seacock open in the warmer months and leave it shut all the time during the winter months. The overboard discharge is never opened except to periodically cycle the valve for maintenance.

My cockpit drains, and sink drains are standpipes with no valve. The cockpit drains are above the waterline.

I do admire your "submarine level" of paranoia. Cycling nearly 20 seacocks on a frequent basis sounds like a chore.